Since liquid crystal displays have features of being thin, low in power consumption and others, the use thereof has been expanding in various articles from large-sized displays to portable information terminals and the development thereof has been actively made. Conventionally, for liquid crystal displays, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (TFTs) are used in TN, and others have been developed and made practicable. However, since nematic liquid crystal is used therein, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
Ferroelectric liquid crystal (FLC) exhibits a very fast response in order of microseconds, and thus FLC is a liquid crystal suitable for high-speed devices. About ferroelectric liquid crystal, there is well known a bistable liquid crystal which has two stable states when no voltage is applied thereto and is suggested by Clark and Lagerwall (FIG. 10 upper graph). However, the liquid crystal has a problem that the liquid crystal has memory properties but gray scale display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto as a liquid crystal making it possible to attain gray scale display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (Non-Patent Document 1, FIG. 10 lower graphs). As the liquid crystal showing the mono-stability, ferroelectric liquid crystals having the phase change of cholesteric phase (Ch)-chiral smectic C phase (SmC*) without passing through the smectic A (5 mA) phase in the temperature lowering process are generally used (FIG. 2 upper part).
On the other hand, as the ferroelectric liquid crystal, there is a material having the phase change of cholesteric phase (Ch)-smectic A phase (5 mA)-chiral smectic C phase (SmC*) so as to exhibit the SmC* phase via the SmA phase in the temperature lowering process (FIG. 2 lower part). Among the ferroelectric liquid crystal material reported so far, most of them are those having the latter phase sequence of passing through the SmA phase compared with the former material which does not pass through the SmA phase. It is known that the latter ferroelectric liquid crystal having the phase sequence of passing through SmA phase in general has two stable states with respect to one layer normal line so as to show the bi-stability.
The ferroelectric liquid crystals have higher molecular orderliness than nematic liquid crystals and it is therefore difficult to align these ferroelectric liquid crystals. Particularly, in ferroelectric liquid crystals obtained by not passing through the SmA phase, two regions differing in the direction of the layer normal line (hereinafter referred to as “double domain”) are generated (Upper part in FIG. 2). Such a double domain brings about white-black inverted displays when the crystal display is driven (FIG. 3), giving rise to a large problem. On the other hand, in ferroelectric liquid crystals obtained by passing through the SmA phase, the layer interval of the smectic layer shrinks during the course of phase change and therefore, the smectic layer has a bent chevron structure to compensate the volume changed by the shrinkage. A domain differing in the direction of the long axis of a liquid crystal molecule depends on the bending direction, causing easy generation of alignment defects called zigzag defects and hairpin defects. Such defects are causes of deteriorated contrast due to light leakage.
In recent years, the One prop Fill (ODF) method has attracted remarkable attention as the method of sealing a liquid crystal. In this method, a sealing agent is coated in frame form in such a manner as to surround a liquid crystal sealing area on one of a pair of substrates, a liquid crystal is dropped on the substrate, then both substrates are laminated on each other in the condition that the pressure between both substrates is decreased sufficiently to bind both substrates with a sealing agent interposed therebetween. The One prop Fill method has the advantage that the time required for the liquid crystal sealing process can be more significantly reduced as compared with the usual vacuum injection method.
As the process for producing a liquid crystal display in the One prop Fill method using a ferroelectric liquid crystal, for example, a method is disclosed in which a liquid crystal solution prepared by dissolving the ferroelectric liquid crystal and a non-liquid crystalline polymer material is coated to one substrate, then, solvents are distilled to form a ferroelectric liquid crystal layer and subsequently, the non-liquid crystal polymer material in the ferroelectric liquid crystal layer is cured (see Patent Document 1). According to this method, since a cured product of the non-liquid crystal polymer material exists as a reinforcing material in the ferroelectric liquid crystal layer, aligning disorder caused by external pressure or the like can be suppressed. However, in Patent Document 1, there is no description concerning the methods of suppressing alignment defects such as a double domain, zigzag defects and hairpin defects.
Further, for example, a method is disclosed in which a mixed solution of a ferroelectric liquid crystal and a polymerizable monomer is coated on one of the substrates to polymerize a polymerizable monomer (see, Patent Document 2 and Patent Document 3). According to this method, the polymerizable monomer molecules are aligned while also, a complex of the ferroelectric liquid crystal and polymerizable monomer is stretched in network form into an anisotropic material, whereby the alignment of the liquid crystal molecules is promoted, making it possible to limit the generation of alignment defects such as zigzag defects. Moreover, like the above method, this method can suppress aligning disorder caused by external pressure.
Patent Document 1: Japanese Patent Application Laid-Open No. 7-199162
Patent Document 2: Japanese Patent Application Laid-Open No. 2000-122043
Patent Document 3: Japanese Patent Application Laid-Open No. 2006-234885
Non-Patent Document 1: NONAKA, T., Li, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599.